Because all tabs except first one are set to display none, Gmedia can’t calculate the size of the element in which the gallery should be rendered.
You can go to Appearence -> Edit -> and add code below to theme’s Stylesheet (style.css) file:
.osc-res-tab .tab-content { position:relative; padding:0 !important; }
.osc-res-tab .tab-content > .tab-pane { display:block !important;visibility:hidden;position:absolute;z-index:-100;pointer-events:none;width:100%;padding:10px; }
.osc-res-tab .tab-content > .active { display:block;visibility:visible !important;position:relative !important;z-index:initial !important;pointer-events:auto !important; }